Guagua Pichincha

Ecuador

0.171°S, 78.598°W; summit elev. 4784 m

All times are local (unless otherwise noted)

The central dome of pale gray lava, surrounded by a flat area covered with a fine tephra powder ~10 cm thick, was observed on 26 July (figure 3). An area of fumaroles was observed on the central dome (figure 4, top) where numerous vents, to 30 cm in diameter, were emitting white steam-rich gas with a strong sulfurous smell. Some yellow sulfur deposition was occurring. Another area of fumaroles was observed on the S wall of the crater (figure 4, bottom) where individual vents were smaller than those on the dome, but were reaching higher temperatures. Some vents were emitting hot water, while others had pools of boiling mud. Gases from the S-wall fumaroles were the same as those emitted from the central dome fumaroles. The hottest vent was also the loudest.

Figure 3. Sketch map of Guagua Pichincha's central crater, August 1990, from an INEMIN geological map prepared by Franco Barberi and others. Courtesy of Sean Hodges, University of Oxford.

Figure 4. Guagua Pichincha's fumarole locations (solid circles) and temperatures, July 1990, on the lava dome (top) and SE crater floor (bottom). See figure 3 for fumarole field locations within the crater. Courtesy of Sean Hodges.

During the late afternoon of 29 July, geologists heard loud explosions from the direction of Guagua Pichincha while on Imbabura, ~70 km N of the volcano.

Geologic Background. Guagua Pichincha and the older Pleistocene Rucu Pichincha stratovolcanoes form a broad volcanic massif that rises immediately to the W of Ecuador's capital city, Quito. A lava dome is located at the head of a 6-km-wide breached caldera that formed during a late-Pleistocene slope failure ~50,000 years ago. Subsequent late-Pleistocene and Holocene eruptions from the central vent in the breached caldera consisted of explosive activity with pyroclastic flows accompanied by periodic growth and destruction of the central lava dome. One of Ecuador's most active volcanoes, it is the site of many minor eruptions since the beginning of the Spanish era. The largest historical eruption took place in 1660, when ash fell over a 1000 km radius, accumulating to 30 cm depth in Quito. Pyroclastic flows and surges also occurred, primarily to then W, and affected agricultural activity, causing great economic losses.